There is known a method to separate a raw fuel to each of a high-octane number fuel and a low-octane number fuel by a separation device, and to selectively provide the separated fuel to an internal combustion engine (Japanese Patent Laid-open Publication No. 2007-278298 hereinafter referred to as Patent Document 1 and Japanese Patent Laid-open Publication No 2009-203909 hereinafter referred to as Patent Document 2). A housing of the separation device is sectioned into two sections by a separation membrane, and one of the two sections is decompressed by a vacuum pump on an occasion of fuel separation.